The invention described herein may be manufactured and used by or for the government of the United States of America for governmental purposes without the payment of any royalties thereon or therefore.
1. Field of the Invention
The present invention relates in general to reduced toxicity hypergolic bipropellants and, more particularly, to the development of new rocket fuels which are hypergolic with 70-100 weight % hydrogen peroxide. These novel bipropellant combinations are less toxic than conventional hydrazine based bipropellant systems.
2. Description of Related Art
Innovative propellants have long been used by the United States Navy for power generation, propulsion and ordnance. Prime considerations in the post World War II era have been specific impulse, volumetric energy content, surge/mobilization readiness and shipboard safety. While these parameters are still important, toxicity, personnel endangerment, environmental concerns, commercial transitions and cost have been added to the list of considerations to be taken into account.
Traditional propulsion systems requiring storable hypergolic bipropellants have used a hydrazine based fuel, such as monomethyl hydrazine, combined with nitrogen tetroxide or inhibited red fuming nitric acid. Storable hypergolic bipropellant systems using interhalogen oxidizers have also been developed and used. However, these oxidizers are highly toxic to humans and extremely reactive. As a result, these oxidizers are difficult to use and dangerous in many environments. Further, these bipropellants pose significant environmental problems and have high associated costs. When a mission requires the use of a storable hypergolic bipropellant, the choice has traditionally been limited to a hydrazine based fuel used in conjunction with N2O4 or IRFNA. These bipropellants deliver excellent performance, but they are highly reactive, toxic and/or carcinogenic substances. It is very difficult and expensive to implement these bipropellants into propulsion systems, which are able to satisfy stringent military safety requirements.
In the past, various polar and nonpolar fuels have been used with hydrogen peroxide to form usable bipropellants. However, except for a potassium cuprocyanide catalyzed hydrazine hydrate fuel discovered in the 1940""s, these bipropellants were not hypergolic and required an ignition system. These non-hypergolic bipropellants are not well-suited for applications requiring pulse mode operation and/or multiple restarts. Thus, traditional hydrogen peroxide based bipropellants could not be considered as viable replacements for the more toxic hypergolic bipropellants. The potassium cuprocyanide catalyzed hydrazine hydrate fuel yielded poor performance compared to nitrogen tetroxide/monomethyl hydrazine and is also a highly toxic fuel. Thus, it has never been a serious fuel candidate.
During the last decade the, the propulsion community launched a significant research effort to develop a new, less toxic, alternative storable hypergolic bipropellants for divert and attitude control systems (DACS) that would meet shipboard requirements. Rocket grade (90-99%) hydrogen peroxide (RGHP) emerged as the liquid oxidizer of choice. The problem was a general lack of hypergolic fuels that could be used successfully with the RGHP. The first significant breakthrough was the development of a manganese acetate catalyzed methanol fuel sufficiently hypergolic to yield successful rocket engine firings.
Please refer to U.S. Pat. No. 5,932,837 issued Aug. 3, 1999 to Rusek, et al. The formulations of the U.S. Pat. No. 5,932,837 provide a reduced toxicity hypergolic miscible fuel, which can be used in combination with a rocket grade hydrogen peroxide to form a reduced toxicity miscible bipropellant with rapid ignition capabilities. In addition, the formulations of the U.S. Pat. No. 5,932,837 provide a reduced toxicity hypergolic miscible fuel containing a dissolved manganese compound, which forms a catalyst in solution. This new bipropellant is especially applicable for use in divert/attitude control systems, as well as general propulsion applications.
The concept for the formulations of the U.S. Pat. No. 5,932,837 was derived from earlier research involving catalyst-doped JP fuels, specifically JP-10. These fuels were formulated by dissolving a manganese containing organic compound in JP-10 fuel. The manganese catalyzed JP-10 fuels would initiate decomposition of the RGHP when sprayed together in a rocket motor injector, but satisfactory hypergolic ignitions were not obtained. The nonpolarity of the JP-10 and the polarity of RGHP did not enable mixing of the two propellants in a manner intimate enough or fast enough to attain the degree of hypergolicity required. In addition, theoretical energy calculations of the combination of the fuels and RGHP resulted in an optimum oxidizer to fuel ratio in the range 5:1 to 7:1. The next step was to find a catalytically active manganese, or other transition metal compound, which was soluble in a lower molecular weight polar solvent that could also serve as the fuel. The result was a hypergolic fuel composition consisting of manganese acetate tetrahydrate dissolved in methanol.
Unfortunately, the formulations of the U.S. Pat. No. 5,932,837 exhibited poor aging/storage characteristics. In time, irreversible degradation occurred in the fuel making it unacceptable for long-term tank storage. The formation of the precipitate is accelerated by heat and by the presence of water and oxygen. This is a fatal flaw for the hypergolic methanol/manganese acetate fuel described in the U.S. Pat. No. 5,932,837.
U.S. patent application Ser. No. 09/510,993, incorporated herein by reference, provides a reduced toxicity hypergolic bipropellant consisting of a modified methanol/manganese acetate fuel and rocket grade hydrogen peroxide oxidizer. The modified fuel resists the formation of precipitate over time when exposed to upper limit thermal environments. This is accomplished by buffering the pH of the fuel with acetic acid and alkali acetate and the addition of a polar amide species to increase the polarity of the solvent. Although the formulations of the Ser. No. 09/510,993 application resist the formation of precipitate better than the formulations of the U.S. Pat. No. 5,932,837, the resultant fuels do not have sufficient stability to satisfy Navy requirements. Therefore, a need in the art exists for rocket fuels with superior performance and acceptable long term tank storage capability
A preferred embodiment of the present invention provides a hypergolic fuel, either a reactive fuel or a catalytic fuel, which is used in combination with rocket grade hydrogen peroxide oxidizer to produce a reduced toxicity hypergolic bipropellant. The rocket grade hydrogen peroxide oxidizer comprises about 70 weight % to about 99 weight % of hydrogen peroxide with the balance water. The preferred hydrogen peroxide concentration is about 94 weight % to about 99 weight %. The reactive fuel is comprised of about 6 weight % to 10 weight % reducing agent dissolved in a fuel, which is also a solvent for the reducing agent. The resultant reactive fuel is intrinsically reactive with the rocket grade hydrogen peroxide oxidizer. The catalytic fuel is comprised of about 6 weight % to about 10 weight % catalytic agent dissolved in a fuel, which also serves as a solvent. The resultant catalytic fuel stimulates the decomposition of the rocket grade hydrogen peroxide oxidizer upon contact.
One object of a preferred embodiment of the present invention is to provide a reduced toxicity hypergolic fuel, which can be used in combination with rocket grade hydrogen peroxide to form a reduced toxicity hypergolic bipropellant with rapid ignition capabilities.
Another object of a preferred embodiment of the invention is to provide a reduced toxicity hypergolic fuel, which remains stable when subjected to long-term storage at maximum and minimum service temperatures.
Another object of a preferred embodiment of the invention is to provide a reduced toxicity hypergolic fuel, which is not carcinogenic and less toxic than hydrazine based fuels. In addition, it is preferred that the hypergolic fuels not be mutagens or teratogens.
A further object of a preferred embodiment of the invention is to provide a reduced toxicity hypergolic fuel formulation, which eliminates the formation of precipitates during long-term tank storage at or near the upper service temperature and provide a fuel which has an ignition delay time of less than 10 milliseconds in a low pressure environment, less than 1 psi.
A still further object of a preferred embodiment of the invention is to provide a fuel, which has a low vapor pressure/high flash point, i.e. a flash point greater than 120xc2x0 F. The low vapor pressure/high flash point reduces flammability hazards and personnel exposure hazards during handling or in the event of a spill.
A still further object of a preferred embodiment of the invention is to provide a fuel, which has a freezing point lower than the freezing point of RGHP.
A still further object of a preferred embodiment of the invention is to provide a fuel, which is formulated to be miscible with water to ease firefighting complications.
A still further object of a preferred embodiment of the invention is to provide a fuel, which is chemically stable over a wide temperature range for an indefinite period of time and resists changes in chemical composition and/or the formation of residues, crystals or precipitates over time in a temperature range of xe2x88x9240xc2x0 F. to 140xc2x0 F.
A still further object of a preferred embodiment of the invention is to provide a fuel, which delivers an Isp greater than 230 seconds with 98% RGHP at a chamber pressure of 500 psi expanded to 14.7 psi.
A still further object of a preferred embodiment of the invention is to provide a fuel, which is not detonable or prone to adiabatic compression and has sufficient thermal stability so that explosions will not occur due to regenerative cooling and/or injector head thermal soak-back experienced in on-off-on operation.
Not Applicable.
The reduced toxicity hypergolic bipropellant of a preferred embodiment of the present invention contains a hypergolic fuel and rocket grade hydrogen peroxide oxidizer. The hypergolic fuel may be a reactive fuel or a catalytic fuel. The hydrogen peroxide oxidizer consists of about 70 weight % to about 99 weight % H2O2, more preferably rocket grade hydrogen peroxide which is about 90 weight % to 99 weight % H2O2, and most preferably about 94 weight % to 99 weight % H2O2. The reactive fuel is comprised of about 6 weight % to 10 weight % reducing agent dissolved in a fuel, which is also a solvent for the reducing agent. The catalytic fuel is comprised of about 6 weight % to about 10 weight % catalytic agent dissolved in a fuel, which also serves as a solvent.
The reactive fuel of a preferred embodiment of the present invention is produced by dissolving a reducing agent in a solvent that is also a fuel. Reactive fuels contain ingredients that are intrinsically reactive with rocket grade hydrogen peroxide oxidizer. Upon contact with rocket grade hydrogen peroxide oxidizer, reactive fuels react vigorously with the hydrogen peroxide resulting in ignition. The catalytic fuel is produced by dissolving a catalytic agent in a solvent that is also a fuel. Catalyzed fuels contain a catalyst, which, upon contact/mixing with the rocket grade hydrogen peroxide oxidizer, promotes rapid catalytic decomposition of the rocket grade hydrogen peroxide liberating highly reactive monotomic oxygen and heat. Hot oxygen in intimate contact with fuel results in rapid ignition.
Reactive fuels are fuels that are, or contain, strong reducing agents. Since rocket grade hydrogen peroxide is a strong oxidizing agent, an immediate and energetic reaction occurs upon contact. Rocket grade hydrogen peroxide reacts instantly and violently with most hydrides. Rocket grade hydrogen peroxide also reacts vigorously with many other substances, which have strong reducing properties. The preferred hydrides are lithium borohydride, sodium borohydride and postassium borohydride. Sodium borohydride is the most preferred reducing agent. Other applicable reducing agents include thiosulfates, thiocyanates and cyanides. Also, lithium metal dissolved in amines and/or ammonia can serve as a reactive fuel. Oraganoaluminum or other organometallics could also be used. It is recognized that a wide variety of organic solvents/fuels can be used to dissolve reducing agents to form a reactive fuel that is hypergolic with RGHP However, sodium borohydride is the preferred reactive additive.
In a preferred embodiment, 1.5 to 2.0 moles of reducing agent is dissolved in one liter of solvent. Typical solvents include tri(ethylene glycol)dimethyl ether (triglyme), diglyme, dimethylaminoethylazide and diethylenetriamine. Other amines, ethers or organic compounds resistant to reduction could be used. In a preferred embodiment, the hypergolic bipropellant of the present invention has an oxidizer-to-reactive fuel ratio in the range of about 3:1 to about 4.5:1.
In a preferred embodiment, the reactive fuel is at least partly miscible with hydrogen peroxide. Most preferably, the reactive fuel is fully miscible with hydrogen peroxide. Miscibility is advantageous so that RGHP can penetrate (intimately mix with) the reactive fuel quickly and make rapid contact with the reducing agent. If the reactive fuel is hydrophobic, rapid and smooth ignition is difficult to attain. However, an injector design with intense shear mixing may overcome this limitation.
Catalyzed fuels are fuels that contain a catalytic agent that rapidly stimulates the decomposition of hydrogen peroxide upon contact. The decomposing peroxide releases monoatomic oxygen, which exists for a brief moment until diatomic oxygen is formed. The decomposing peroxide also releases a significant amount of heat. Oxygen, heat, and fuel are the required elements for ignition. Catalysts are typically metal atoms such as, but not limited to Mn, Co, V, Ag, Cr, Pt, Ru, Pd, Fe, Ni, and Cu. Some of these elements are highly catalytic in the elemental state. Some compounds containing these elements are highly catalytic, some are moderately catalytic, and others are not catalytic to any appreciable extent. It is also recognized that some compounds that do not contain any metal atoms are catalytic. Catalytic agents include manganese acetate (anhydrous or hydrate), manganese butyrate (anhydrous or hydrate), manganese octoate, manganese acetylacetonate, cobalt octoate, cobalt butyrate (anhydrous or hydrate), cobalt acetate (anhydrous or hydrate), cobalt acetylacetonate, dicyclopentadienyliron, copper ethylhexoate and potassium dicyanocuprate. Although many transition metal containing compounds may serve as effective catalysts, the preferred catalysts are carboxylic salts of transition metal compounds.
In a preferred embodiment, the solvent, which also acts as a fuel, dissolves enough catalytic agent to assure immediate hypergolic ignition. Preferably, a minimal amount of metal catalyst is used. Preferred solvents include dimethylaminoethylazide, diethylformamide, dimethylaminoethanol, and diethylaminoethanol. Other solvents may include other amines, hydrocarbons such as JP-10, nitrated organic compounds, such as nitromethane or nitropropane, and alcohols. In a preferred embodiment, the hypergolic bipropellant of the present invention has an oxidizer-to-catalytic fuel ratio in the range of about 2.5:1 to 4.5:1.
In a preferred embodiment, the fuel is a good solvent for the catalytic substance. It is advantageous that the resultant solution is stable over the service temperature range and that no degradation occurs with time. It is advantageous that the fuel be at least partly miscible with hydrogen peroxide. Most preferably, the catalytic fuel is fully miscible with hydrogen peroxide. Again, miscibility is important so that the RGHP can penetrate (intimately mix with) the fuel to maximize contact with the dissolved catalytic material. Hydrophobic fuels greatly impede the catalytic action of the dissolved catalyst.